Mapping the local and brain-wide network effects by optogenetic activation with an MRI-guided robotic arm

Abstract

The optogenetically driven manipulation of circuit-specific activity has been very successful to enable functional causality studies in animals, but its global effect on the brain is rarely reported. Optical fiber-mediated optogenetic activation and neuronal Ca2+ recording in combination with fMRI provide a multi-modal fMRI platform with cross-scale brain dynamic mapping schemes, which can elucidate network activity upon circuit-specific optogenetic activation. However, despite highly promising prospects in animal brain research, there are still methodological and conceptual deficiencies, e.g., off-target effects and antidromic activity effects, which remain challenging for the current state of the art. To overcome these difficulties, this thesis describes two technical advances applied at the multi-modal fMRI platform, bridging the methodological and conceptual gap in optogenetics, brain function and animal behavior.

First, an MRI-guided robotic arm (MgRA) is developed to increase the target accuracy for optogenetic manipulation or microinjection at the multi-modal fMRI platform, merging fMRI with concurrent deep brain optogenetics in rats. The 4-degrees-of-freedom MgRA allows high precision (50 μm per step) and sufficient mobility range (10 mm in the ventral-dorsal, rostral-caudal and medial-lateral directions) to manipulate fiber optic or injection needles into the brain in real time and provide high flexibility for multi-site targeting along the trajectory, which shows a clear advantage over the standard stereotaxic-based implantation strategy.

Second, the multi-modal fMRI platform provides a specific calcium amplitude-based correlation analysis to study corpus callosum (CC)-mediated brain-wide network dynamics with taking antidromic activity effect into consideration. Since the callosal fibers are reciprocally projecting to two hemispheres, bilateral ortho-vs. antidromically evoked neural activity is difficult to disentangle. Here we not only detected strong antidromic activity, but also detailed temporal dynamics through CC-mediated orthodromic inhibitory activity. The calcium amplitude-based correlation map was created to reveal the brain-wide inhibitory effects from the CC-specific optogenetic stimulation.

Last, this multi-modal fMRI platform was used to acquire the optogenetically driven neuronal Ca2+ with single-vessel BOLD and cerebral-blood-volume weighted signal from individual venules and arterioles, respectively, in the hippocampus. We characterized distinct spatiotemporal patterns of hippocampal hemodynamic responses that were correlated to the optogenetically evoked Ca2+ events and further demonstrated the significantly reduced neurovascular coupling (NVC) efficiency upon spreading depression-like Ca2+ events. These results provide a direct measure of the NVC function at varied hippocampal states in animal models.

Overall, the technical advances described in this thesis demonstrate the powerful multi-modal fMRI platform to map, analyze and characterize the dynamic brain function across multiple scales and underscore the caution to interpret circuit-specific regulatory mechanisms underlying behavioral or functional outcomes with optogenetic tools.